Thermostatic mixing valves provide a source of water having a desired temperature and are operable to maintain the desired water temperature substantially constant. Typically, the desired water temperature is obtained by controlling the relative proportions or hot and cold water admitted to a mixing chamber and adjusting the relative proportions to maintain the desired water temperature substantially constant.
The known thermostatic mixing valves employ an actuator responsive to water temperature for adjusting the relative proportions in which the hot and cold water are mixed to maintain the desired water temperature substantially constant. Various types of actuators for providing the thermal control of the water temperature are known including thermally responsive elements positioned in the water flow for actuating the valve in response to the detected water temperature. For example, wax capsules or bimetal or memory metal type actuators. Alternatively, a motor may be provided to actuate the valve in response to the water temperature detected by temperature sensors.
In use, the outlet water temperature can deviate from the desired temperature if the temperature and/or pressure of one or both of the hot and cold water supplies to the mixing valve changes. A sudden increase or decrease in temperature that is sufficient to be discernible to the user may result in an uncomfortable experience. As a result, steady state temperature performance requirements are becoming increasingly more stringent with reductions in the permitted temperature deviations being introduced. For example, in mixing valves for healthcare applications such as in hospitals or care homes for the elderly or disabled, temperature deviations of only a few degrees are permitted.
In addition to steady state temperature performance requirements, transient temperature performance requirements dealing with temperature overshoots or undershoots when the operating conditions suddenly change are increasingly being included in valve specifications for certain applications, especially in the healthcare market. Transient temperature changes typically arise when the desired water temperature is changed, for example from cold to hot or where the valve is initially turned on. Under these conditions, the valve may initially change the relative proportions of hot and cold water more than is required before settling to produce water having the new desired temperature. The size and duration of any temperature overshoot or undershoot may only last a few seconds but is again discernible to the user if more than a few degrees and can be uncomfortable even if not presenting a safety risk.
A common approach to meet these tighter performance standards has been to seek to improve the thermal control system and in particular the accuracy and speed of response of the system to detected changes in the desired temperature of the water. This approach is based on the assumption that the water has been properly mixed so that the system responds equally to any changes tending to increase or decrease the desired water temperature.
This approach has not been completely successful, however, and in many cases valve performance is generally not as good as predicted by theoretical calculations. Often the valve responds more to changes in one water supply than the other and the thermal control system is adapted by trial and error to produce an arrangement in which the response is consistent to changes in either supply. In particular, the outlet water temperature may deviate from that selected if the inlet pressures change and skewing the response of the valve to inlet pressure changes may be required to ensure that any deviation of the outlet water temperature fits within a permitted tolerance range. Such skewing of the response is undesirable however as it may not result in optimum performance for all the circumstances that may arise in use.
As a result of extensive testing, we have now found that in many existing valve designs incomplete mixing of the water occurs and the water temperature detected by the thermal control system is made up of the temperature of partially mixed and unmixed streams of hot and cold water. Indeed, for some designs, as little as 25% of the water stream is made up of mixed water having the desired temperature.
Typically, the waterways within the valve are made up of spaces between valve components and are not particularly streamlined. This can produce variations in the flow through the valve which, together with incomplete mixing of the water, is now believed to be a reason for quite significant variations in performance occurring from one valve to another. For example, we have found that when a valve that fails to meet the required performance standards during testing is taken apart, there is often nothing wrong with it and, when re-assembled with the same components, the valve can pass the performance standards on re-testing.
As a result, a considerable amount of time and attention has been spent in ensuring that only valve components of the highest quality are used and that assembly is carried out very carefully. This adds considerably to production costs and the fundamental problem of variations in performance between valves assembled from the same components still persists.
The present invention has been made from a consideration of the aforementioned disadvantages and drawbacks of existing thermostatic mixing valves.